Optical system of a microlithographic projection exposure apparatus

ABSTRACT

In a method for improving imaging properties of an illumination system or a projection objective of a microlithographic projection exposure apparatus, which comprises an optical element having a surface, the shape of the surface is measured directly at various points. To this end, a measuring beam is directed on the points, and the reflected or refracted beam is measured, e.g. using an interferometer. Based on deviations of the measured shape from a target shape, corrective measures are derived so that the imaging errors of the optical system are improved. The corrective measures may comprise a change in the position or the shape of the optical element being analyzed, or another optical element of the optical system. The target shape of the surface may, for example, be determined so that the optical element at least partially corrects imaging errors caused by other optical elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/190,555 filed Jul. 27, 2005 which claims priority of German patentapplication 10 2004 037 278.0, filed Jul. 31, 2004 both priorapplications full disclosures are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to optical systems, for example projectionobjectives or illumination systems, of microlithographic projectionexposure apparatuses as are used for the production of microstructuredcomponents. The invention also relates to a method for improving imagingproperties of such optical systems.

2. Description of Related Art

Microlithographic projection exposure apparatuses, which are used forthe production of large-scale integrated electrical circuits and othermicrostructured components, contain an illumination system which is usedto generate a projection light beam. The projection light beam isdirected at a mask, which contains minute structures to be imaged and isarranged in an object plane of a projection objective. The projectionobjective forms a reduced image of the structures contained in the maskon a photosensitive layer, which is located in an image plane of theprojection objective and may, for example, be applied on a wafer.

Owing to the small size of the structures to be imaged, very stringentrequirements are placed on the imaging properties of the projectionobjective, and increasingly of the illumination system. Imaging errorsof these optical systems must therefore be reduced to a tolerable levelby suitable measures.

In this context, it has been known for a long time to change theposition of individual optical elements inside the relevant opticalsystem with the aid of manipulators. Such position changes, however, canonly correct a few imaging errors retrospectively. Further imagingerrors can be corrected by changing the shape of optical elements, ormore precisely their reflective or refractive surfaces.

In connection with lenses, for example, it is known from U.S. Pat. No.6,388,823 B1 assigned to the applicant to bend a lens withoutsignificantly changing its thickness. To this end, the lens is engagedcircumferentially by a plurality of actuators which generate theintended bending moments in the lens.

EP 1 376 192 A2 discloses a projection objective of a microlithographicprojection exposure apparatus, which is constructed exclusively usingmirrors. Two of the mirrors can be deformed so that their reflectingsurfaces respectively change in shape. Various configurations ofactuators which can provide an intended deformation of the mirrorsurface are also described.

Optical elements with deliberately deformable surfaces, which are oftenalso referred to as active or adaptive optical elements, are alsosuitable for correcting those imaging errors which vary as a function oftime. For example, there are imaging errors which are due to changes inrefractive index and/or shape, which are in turn a result of heatproduced in the optical elements by the projection light. The energeticprojection light can furthermore lead to irreversible materialmodifications at those places on the optical elements which are exposedto the projection light. It is also known that the shape of opticalelements can even change because of settling and relaxation effects whenthey are not exposed to projection light.

When time-variable imaging errors, attributable to the aforementioned orsimilar causes, are intended to be corrected by changing the shape ofindividual surfaces during operation of the projection exposureapparatus, then the corrective measures must be designed so that theycan be implemented as much as possible in short exposure pauses.Corrective measures which require longer down times of the apparatusreduce the throughput and therefore compromise their economic viability.

In connection with projection objectives, it has therefore been proposedto analyze the imaging properties of the projection objective duringexposure. It is known, for example from US Patent Application2003/0002023 A1 assigned to the applicant, to couple a measuring lightbeam into the projection objective so that it lies outside the actualprojection light beam after it emerges from the projection objective.The emergent measuring light beam is analyzed with the aid of awavefront detector, so that it is possible to infer the imagingproperties at least of that part of the projection objective throughwhich the measuring light beam has passed. On the basis of thesemeasurements, corrective measures can then be determined which interalia may comprise changing the shape of adaptive lenses or adaptivemirrors. A similar method is also known from EP 1 376 192 A2, which wasalready mentioned above.

Such known measuring methods, however, can quantitatively register onlysome specific imaging errors. The causes explained above may, however,also induce time-variable imaging errors which cannot be analyzed duringprojection operation by the known method.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an optical systemof a microlithographic projection exposure apparatus having improvedcorrection possibilities. It is also an object of the invention toprovide a method by which the imaging properties of such a system can beimproved further.

An optical system which achieves this object comprises an opticalelement that has a surface which is exposed to projection light during aprojection. According to the invention, a measuring device is providedthat determines positional data of at least one point on the surface. Ifpositional data are determined for more than that one point, it ispossible to compute the shape of the surface. The more points that areincluded in the determination, the better the computed shape correspondsto the actual shape of the surface. The positional data may comprisecoordinates of the point relative to a reference system, for example, ordistances to a point of reference.

The invention is based on the discovery that the optical effect of anoptical element can be determined optimally by directly determining theshape of an optically active surface of the optical element with a highaccuracy. If it is found when determining the shape of the surface thatthe deviations from a target shape exceed a predetermined amount, thensuitable corrective measures may be instigated. These may, for example,comprise using suitable manipulators which are known as such to changethe position or shape of the optical element being analyzed or, if thisis not possible or not expedient for other reasons, another opticalelement.

According to previously known methods, it is possible to determine theimaging properties of the overall optical system within certain limitsby processing the images obtained with a measuring light beam which haspassed through a multiplicity of individual optical elements. With theseknown methods, however, it is often difficult or even impossible tolocate the causes of particular imaging errors and, in particular, inferthe deformation of individual surfaces. This is a disadvantage becausemany imaging errors can only be corrected sufficiently by knowing whichoptically active surfaces exhibit deviations from a target shape. Incontrast, the method according to the invention makes it possible todirectly determine deformations of virtually any optically activesurfaces, which offers considerably improved correction possibilities.

The target shape, with which the measured shape of the surface iscompared, may for example be determined so that the optical elementcauses minimal imaging errors. This is generally the shape on which thedesign of the optical system was based.

As an alternative to this, the target shape may also be determined sothat the optical element at least partially corrects imaging errorswhich are caused by other optical elements, and which may possibly notoccur until operation of the exposure apparatus. For example, if theoptical element is a concave mirror in a catadioptric projectionobjective, which is arranged in or close to a pupil plane of the system,then many time-variable imaging errors can be substantially corrected ata central point in the projection objective by selectively changing theshape of the surface of the concave mirror.

The shape of the surface being analyzed may be changed with the aid ofmanipulators. In this case, the manipulators may be driven so thatdeviations of the measured shape from the target shape lie below apredeterminable threshold value. To this end, the measuring device maybe connected to a control device for at least one manipulator whichaffects the shape of the surface, and to which a target shape of thesurface can be delivered as a guide quantity. In the scope of a controlprocess, it is thus possible to bring the actual measured shape of thesurface closer and closer to the target shape.

In this configuration, the invention therefore makes it possible tomonitor the effect of the manipulators directly on the relevant opticalelement. This is advantageous in so far as the manipulators used forthis purpose do often generate control messages, from which it ispossible to infer which position the manipulators have reached and whichshape the surface should therefore have. However, it is often notpossible to infer the actual shape of the surface accurately enough fromthe control messages alone. The causes of these inaccuracies may, forexample, be drift effects of measuring devices which are assigned to themanipulators.

In general, the optical elements contained in the optical system oftenfall significantly short of their potential to cause imaging errorsbecause of energetic projection light or similar causes. A thinplane-parallel plate, which is arranged in an optical system so thatprojection light passes through it uniformly over a sizeable area, willgenerally induce smaller time-variable imaging errors than, for example,is the case with a strongly curved lens through which the projectionlight passes with a very small beam diameter. In more complex opticalsystems, for example projection objectives in general, it is thereforesufficient to carry out the inventive direct measurement of one or moreoptically active surfaces only on a few optical elements or even asingle optical element.

Furthermore, it is generally unnecessary to record the entire opticallyactive surface precisely in terms of measuring technology. In order toobtain information about changes in the imaging behavior, which arecaused by passive or active shape changes, it is often sufficient toobtain positional data at a few points or even only at a single point onthe surface. When this description refers to measuring the shape of thesurface directly, therefore, this consequently also includes thesituation in which the height of a surface relative to a planeperpendicular to the optical axis is determined only at one or moreindividual points. The term direct measurement thus does not includemeasurements in which the shape of the surface is merely inferredindirectly from the overall optical effect, i.e. by simulation. Instead,direct measurement means to directly obtain positional data, e.g. theheight relative to a plane perpendicular to the optical axis, for one ormore individual points on the surface.

An interferometric measuring device, in which a measuring beam thatstrikes the surface interferes with a reference beam that does notstrike the surface, is particularly suitable for analyzing the surface.Interferometric measuring devices can achieve very high measurementaccuracies; such measuring devices furthermore allow a compactstructure, so that they can also be integrated in projection objectiveswith their spatial limitations.

An interferometric measuring device typically comprises a light sourcefor generating coherent light and an interferometer. The wavelength ofthe light used is preferably independent of the wavelength of theprojection light, so that the light generated by the light source doesnot itself lead to exposure of the photosensitive layer applied on thewafer, even if a small part of the light used for the measurement entersthe projection light beam path by scattering or the like. In principle,it is also possible to extract a part of the projection light which isgenerated by the laser contained in the illumination device, and to usethis for the interferometric analysis of the surface. This is suitableparticularly when the relevant optical element is arranged in theillumination device of the projection exposure apparatus.

When selecting the wavelength of the measuring light it should also beborne in mind that the surface to be analyzed must reflect the measuringbeam. For refractive optical elements, which generally have ananti-reflection coating on their surfaces, there are usually wavelengthsat which the anti-reflection coating does not work. If such a wavelengthis selected for the measuring beam, then it is possible to ensure that asufficiently large part of the incident measuring beam will bereflected.

In order to be able to direct the measuring beam at different points onthe surface of the optical element, the measuring device may furthermorecontain at least one tiltable deviating mirror. The deviating mirror maybe tilted continuously or intermittently about one or more spatial axes,so that the measuring beam can be directed at a multiplicity of pointson the surface to be analyzed.

Instead of an interferometric measuring arrangement, it is alsoconceivable to use other optical measuring arrangements which cananalyze surfaces accurately. For example, it is possible to usetriangulation measuring methods and methods which are based on theauto-focus principle, as is known for instance from optical data memoryreaders.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a meridian section through a projection objective of amicrolithographic projection exposure apparatus in a highly schematizedrepresentation, with an interferometric measuring device for analyzing amirror surface;

FIG. 2 shows an enlarged detail of FIG. 1, in which further details ofthe measuring device can be seen.

FIG. 3 is an exemplary schematic flow chart of a method for improving animaging property of an optical system of a microlithographic projectionexposure apparatus.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

FIG. 1 represents a projection objective, denoted in its entirety by 10,of a microlithographic projection exposure apparatus in a simplifiedmeridian section. The projection objective, which is also described inPCT/EP03/04015 filed by the applicant, is used to form a reduced imageof structures, contained in a mask 12, onto a photosensitive layer 14which consists of a photoresist and is applied on a substrate 15. Themask 12 is arranged in an object plane OP and the photosensitive layer14 is arranged in an image plane IP of the projection objective 10.

After passing through the mask 12, the projection light 16 indicated bydots in FIG. 1, which is generated by an illumination system of theprojection exposure apparatus and has a wavelength of 157 nm in theexemplary embodiment represented, travels through a plane-parallel plate18 and a lens L1 into a beam-splitter cube 20. There, the projectionlight 16 is reflected at a polarization-selective beam-splitter layer 22contained in it, and sent through a lens L2, a quarter-wave plate 24 andtwo further lenses L3 and L4 onto a mirror unit 26 explained in detailbelow, which comprises an adaptive mirror with an essentially sphericalmirror surface 28.

After reflection at the mirror surface 28, the projection light againpasses through the lenses L4 and L3, the quarter-wave plate 24 and thelens L2, and strikes the polarization-selective beam-splitter layer 22.There, however, the projection light 16 is not reflected but transmittedsince the polarization direction of the projection light 16 has beenrotated through 90° by passing twice through the quarter-wave plate 24.From the beam-splitter cube 20, the projection light 16 travels via aplane mirror 30 into a purely dioptric part 32 of the projectionobjective 10 in which lenses (not referred to in detail) are arrangedalong an optical axis indicated by 34.

The mirror unit 26, which is shown with further details in FIG. 2,comprises a plurality of manipulators 36 which engage on the rear sideof the mirror surface 28, not exposed to the projection light 16, sothat the latter can be deformed in a controlled way. The forces requiredfor this may, for example, be generated pneumatically, hydraulically orwith the aid of piezo elements. Further details of suitable manipulators36 can be found in EP 1 376 192 A2 that has been mentioned above andwhose full disclosure is incorporated herein by reference.

The manipulators 36 are connected to a control device 37, whichdetermines a target shape of the mirror surface 28. In the exemplaryembodiment described here, the target shape is designed so as to atleast partially correct time-variable imaging errors in the projectionobjective 10. The imaging errors which occur in the projection objective10 may, for example, be determined by simulation. This is because theway in which individual optical elements heat up, and therefore changetheir shape, under the effect of the energetic projection light isgenerally known. From this, it is in turn possible to determine imagingerrors which are caused by changes in shape or refractive indexattributable to the heating.

Besides this, it is also possible to determine the time-variable imagingerrors by measurements, as is known per se in the prior art. In thiscontext, reference is made to U.S. Pat. No. 6,388,823 B1, whose fulldisclosure is incorporated herein by reference. Another possible way ofmeasuring the imaging errors is to assign measuring devices at least tothose surfaces which can make significant contributions to time-variableimaging errors, as will be described below with respect to the mirrorsurface 28. These measurement devices allow direct measurement of theshape of the surfaces, from which it is possible to deduce veryprecisely not only the size of the imaging errors but also their causes.

Once the time-variable imaging errors have been determined, thensimulation methods known per se can be used to determine a target shapefor the mirror surface 28, with which at least some of the ascertainedimaging errors are at least partially corrected. The control device 37then drives the manipulators 36 so that the mirror surface 28 obtainsthe previously determined target shape.

In order to ensure that the manipulators 36 actually deform the mirrorsurface 28 so that it obtains the previously determined target shape,the mirror surface 28 is analyzed with the aid of an optical measuringdevice 28 at a few selected points. The measuring device 38 is designedas an interferometric measuring device and contains a laser 40 as thelight source. The laser 40 generates a coherent light beam, which isdirected at a beam splitter 42. The light beam passing through the beamsplitter 42 forms a reference beam 44, which strikes an interferometer45 without having been reflected by the mirror surface 28.

The light beam reflected by the beam splitter 42 forms a measuring beam46, which strikes a mobile deviating mirror 48 and is reflected by it inthe direction of the mirror surface 28. In the exemplary embodimentrepresented, the deviating mirror 48 can tilt about a tilting axisindicated by 50 and is connected to an actuating device 52 for thispurpose. By tilting the deviating mirror 48 about the tilting axis 50,the actuating device 52 can direct the measuring beam 46 onto differentpoints on the mirror surface 28. In FIG. 2, a dashed line 46′ indicatesa measuring beam which has been directed onto the mirror surface 28 bytilting the deviating mirror 48 anticlockwise.

The measuring beam 46 reflected by the mirror surface 28 strikes asecond deviating mirror 54, which can likewise tilt about a tilting axis56. A second actuating device 58, which can tilt the second deviatingmirror 54, is synchronized with the first actuating device 52 so thatmeasuring beams 46 reflected by the second deviating mirror 54 alwaysstrike a stationary deviating mirror 60 while being mutually parallel,and are directed from there onto the interferometer 45.

By synchronous adjustment of the deviating mirrors 48, 54, it istherefore possible to analyze all points on the mirror surface 28 whichlie in the plane of the paper, over a wide range. If it is alsonecessary to analyze points in planes parallel thereto, then it is forexample possible to provide a plurality of pairs of tiltable deviatingmirrors 48, 54, onto which a measuring beam is respectively directed. Asan alternative to this, it is possible to mount the deviating mirrors48, 54 in a mobile fashion so that they can also tilt about a tiltingaxis which lies in the plane of the paper. Since the mirror surface 28is concavely curved, light which is directed onto the mirror surface 28out of or into the plane of the paper by the first deviating mirror 48will always be reflected in the direction of the second deviating mirror54, so that its surface can be kept relatively small.

In the interferometer 45, the reference beam 44 interferes with themeasuring beam 46. If the mirror surface 28 is deformed where themeasuring beam 46 strikes the mirror surface 28, then this leads to adifferent optical path length and therefore to a different relativephase relation between the reference beam 44 and the measuring beam 46.By determining this phase relation in the interferometer 45, it ispossible to measure the height of the mirror surface 28 relative to aplane perpendicular to the optical axis. If the mirror surface 28 isanalyzed at a plurality of positions, then this provides a profile ofthe mirror surface 28 whose support points are those points where themeasuring beam 44 has been reflected by the mirror surface 28.

If the control device 37 finds that there are differences between thetarget shape of the mirror surface 28 and the actual shape of the mirrorsurface 28, as determined with the aid of the measuring device 38, thenit adjusts the manipulators 36 so that the actual shape becomes closerto the target shape. To this end, the control device 37 may contain acontrol unit as is known per se in the prior art. The control unit canmake the actual shape of the mirror surface 28 replicate the targetshape with a sufficient accuracy.

If the mirror surface 28 is not intended to be used to correct imagingerrors which have been caused by other optical elements in theprojection objective 10, then the target shape specified by the controldevice 37 will be the shape on which the design of the projectionobjective 10 was based. If the shape of the mirror surface 28 changesduring operation of the projection exposure apparatus, for example dueto local heating by the projection light, then the control unit 37 willdrive the manipulators 36 so that the undesirable shape changes of themirror surface 28 can be reversed. In this case as well, the measuringdevice 38 makes it possible to find any deviations of the shape whichthe manipulators 36 have actually set for the mirror surface 28 from thetarget shape. The control device 37 then adjusts the manipulators 36 sothat the measured deviations lie below a predeterminable thresholdvalue.

1. An optical system, comprising: a) an optical element having a curvedsurface which is exposed to projection light during a projection, and b)a measuring device that determines positional data of a plurality ofpoints on the surface, wherein the measuring device comprises a movablebeam deviating component which is configured to direct a measuring beamsequentially onto the plurality of points on the surface of the opticalelement, and wherein the optical system is one of an illumination systemand a projection objective contained in a microlithographic projectionexposure apparatus.
 2. The optical system of claim 1, wherein themeasuring device determines the positional data of the plurality ofpoints by directing the measuring beam sequentially on the plurality ofpoints such that it is reflected or refracted at each of the pluralityof points, and measuring a deflection of the reflected or refractedmeasuring beam.
 3. (canceled)
 4. The optical system of claim 2, whereinthe measuring device is an interferometric measuring device, in whichthe measuring beam reflected or refracted at each of the plurality ofpoints interferes with a reference beam that does not impinge on thesurface of the optical element.
 5. The optical system of claim 4,wherein the measuring device comprises a light source for generatingcoherent light and an interferometer.
 6. The optical system of claim 5,wherein the measuring device comprises a beam splitter that splits thelight generated by the light source into the reference beam and themeasuring beam.
 7. The optical system of claim 4, wherein the beamdeviating component contains at least one tiltable deviating mirror. 8.(canceled)
 9. The optical system of claim 1, comprising a manipulatorfor changing the shape of the surface.
 10. The optical system of claim9, wherein the measuring device is connected to a control devicecontrolling the manipulator, said control device being configured toreceive a target shape of the surface as a guide quantity.
 11. Theoptical system of claim 10, wherein the target shape is determined suchthat the optical element causes minimal imaging errors.
 12. The opticalsystem of claim 10, wherein the target shape is determined such that theoptical element at least partially corrects imaging errors caused byother optical elements.
 13. The optical system of claim 1, wherein theoptical element is a concave mirror disposed in or in close proximity toa pupil plane of a catadioptric projection objective.
 14. A method forimproving an imaging property of an optical system comprising thefollowing steps: a) providing an optical system wherein the opticalsystem is one of an illumination system and a projection objectivecontained in a microlithographic projection exposure apparatuscomprising an optical element having a curved surface that is exposed toprojection light during a projection; b) determining positional data ofa plurality of points on the surface by directing a measuring beamsequentially onto the plurality of points on the surface by using amovable beam deviating component positioned outside of the projectionlight; c) determining deviations of the positional data from targetdata; and d) implementing corrective measures such that the imagingproperty of the optical system is improved.
 15. The method of claim 14,wherein the positional data of the plurality of points are determinedby: directing the measuring beam sequentially on the plurality of pointssuch that it is reflected or refracted at each of the plurality ofpoints and measuring a deflection of the reflected or refractedmeasuring beam.
 16. The method of claim 14, wherein the correctivemeasures comprise a change in the position or the shape of the opticalelement.
 17. The method of claim 15, wherein the shape of the opticalelement is changed so that the deviations determined in step c) liebelow a predetermined threshold value.
 18. The method of claim 14,wherein the corrective measures comprise a change in the position or theshape of another optical element contained in the optical system. 19.The method of claim 14, wherein the target data of the surface aredetermined such that the optical element causes minimal imaging errors.20. The method of claim 14, wherein the target data of the surface aredetermined such that the optical element at least partially correctsimaging errors caused by other optical elements contained in the opticalsystem.
 21. The method of claim 15, wherein the positional data of thesurface are determined by an interferometric measuring device, in whichthe measuring beam reflected or refracted at the at least one pointinterferes with a reference beam that does not impinge on the surface.22. The optical system of claim 1, wherein the measuring device isarranged completely outside a beam path of the projection light so thatthe surface of the optical element is not shadowed by the measuringdevice.
 23. An optical system comprising: a) an optical element having acurved surface which is exposed to projection light during a projectionwherein the optical system is one of an illumination system and aprojection objective contained in a microlithographic projectionexposure apparatus; and b) a measuring device producing a measuring beamand comprising at least one tiltable mirror which is configured todirect the measuring beam sequentially onto a plurality of points on thesurface of the optical element so as to determine positional data of theplurality of points on the surface of the optical element.
 24. Anoptical system, comprising: a) an optical element having a curvedsurface which is exposed to projection light during a projection whereinthe optical system is one of an illumination system and a projectionobjective contained in a microlithographic projection exposureapparatus; and b) a measuring device that determines positional data ofa plurality of points on the surface, wherein the measurement device iscompletely arranged outside a beam path of the projection light.
 25. Anoptical system, comprising: an optical element having a curved surfacewhich is exposed to projection light during a projection wherein theoptical system is one of an illumination system and a projectionobjective contained in a microlithographic projection exposureapparatus; a measuring device which is positioned outside a beam path ofthe projection light and which comprises: a light source producingcoherent measuring light, a first tiltable deviating mirror thatselectively directs the measuring light to at least one point on thesurface of the optical element, a second tiltable deviating mirror thatis synchronized with the first tiltable mirror and directs the measuringlight reflected from the at least one point on the surface towards aninterferometer in which the measuring light reflected from the at leastone point on the surface interferes with a reference beam that does notimpinge on the surface of the optical element.